The manufacturing of semiconductor integrated circuits starts with a semiconductor wafer, which is typically a silicon disk, which is patterned by way of photolithography, before being cut apart into dies, each one constituting an individual integrated circuit, which is then packaged for sale. After the patterning, it is frequently desirable to examine sections of the wafer very closely, to determine the process results. Due to the nanometric dimensions of the features this examination is often performed using S/TEMs, which are limited to the examination of specimens having a thickness of less than 100 nm. As a result, it is necessary to extract lamellae from the wafer, for imaging by the S/TEM.
This extraction starts with a nanomachining device which may be as shown in simplified form in FIG. 1 and disclosed in greater detail in U.S. Pat. No. 6,268,608, which is incorporated by reference as if fully set forth herein. Nanomachining device 10 includes a scanning electron microscope (SEM) 12 which is used for viewing extraction sites and a focused ion beam (FIB) 14, which is used to remove wafer material, thereby defining lamellae. A gas injection device 16 may be used in conjunction with FIB 14 or SEM 12, to deposit a selected material. The machining takes place in a vacuum chamber 18. A vacuum load lock 20 facilitates introducing and removing wafers into sample vacuum chamber without opening. Alternatively, a nanomachining device may include a FIB, used for both imaging and machining, but no SEM.
In some instances, the nanomachining device 10 is used to finish the extraction (in situ extraction), typically with direct human control of the FIB 14, which is used to completely separate the lamellae from a wafer 15, and a very fine shaft 22, controlled by a micromanipulator 17, which is used to pick up and deposit the lamellae from the wafer 15 supported on machining stage 23 onto a sample holder referred to as a TEM grid. In some instances of in situ extraction, the lamella is attached to the fine shaft 22 by ion beam-induced deposition and transported to a toothed grid, to which the lamella is attached, again by ion beam-induced deposition, and then the connection between the lamella and the fine shaft is severed.
To introduce a new wafer into vacuum chamber 18, a wafer movement device 24 includes a robot arm 26 for moving wafers into the vacuum lock 20 from a wafer cassette holder 28. An air filtering system 30, maintains low particulate levels in wafer movement device 24, thereby introducing fewer contaminants into vacuum chamber 18, through lock 20.
A suite of support and control equipment 32 interfaces with SEM 12, FIB 14, gas injection device 16 and the shaft 22. Equipment suite 32 is in turn controlled by a computer 34, which feeds and responds to a user monitor and control device 36, permitting a human user to control the process.
The vacuum lock 20, wafer movement device 24 including the robot arm 26, the wafer cassette holder 28, the air filtering system 30 and the user monitor and control device 36 are all considered to be part of the front end 40 of device 10. The front end must be carefully constructed to interface correctly with the vacuum chamber 18 and the equipment inside the vacuum chamber 18. For example, because the SEM 12 and FIB 14 are extremely sensitive to vibrations, chamber 18 floats on four pneumatic cushions 42, (two shown) to minimize the vibration of chamber 18. When arm 26 must load a wafer into or remove a wafer from vacuum lock 20 (which is designed to hold two wafers, to ease flow of wafers into and out of chamber 18), it is necessary that the vacuum lock 20, which is rigidly attached to the walls of chamber 18, be aligned with the front end 40. To do this a special pneumatic or hydraulic cylinder 44 is provided, to move lock into this alignment. Accordingly, communications must synchronize this alignment process and the transfer of wafers.
This in situ lamella removal technique requires more human time and more time at the nanomachining device, than the ex situ technique that will be described below, thereby reducing throughput of this device, which is highly undesirable for a costly, high-throughput device. Nanomachining devices that are currently in the design phase should have a throughput of about 10 to 20 minutes per lamellae, with the extraction of the lamellae potentially adding another 3 to 5 minutes per lamellae. Accordingly, being able to perform the lamellae extraction outside of the nanomachining device (ex situ extraction) could significantly increase throughput.
An ex situ plucker 110 is shown in simplified form in FIG. 2 and described in greater detail in U.S. Pat. No. 8,357,913, which is incorporated by reference as if fully set forth herein. Referring to FIG. 2, a stage 112 supports a wafer (not shown), and an illumination source 114, utilizing a fiber optic bundle 116 provides oblique illumination. An optical microscope 118 permits magnified viewing, and a vacuum shaft 120, controlled by a micromanipulator 122, is used to pluck the lamellae, in a process described below in more detail. A suite of control and support equipment 124 serves and controls the optical microscope 118, illumination source 114 and vacuum shaft 120. In turn a computer 126, which includes a data input assembly, controls suite 124, and a user monitor and control system 128, is fed by and controls computer 126. In one embodiment, the data input assembly of the computer 126 includes additional data ports, such as an Ethernet connection and USB ports. Also, a wafer movement device 130 uses a robot arm 132 to move wafers from a wafer cassette holder 134 to the stage 112. Finally an air filtering system 140, maintains air cleanness in station 110.
Referring to FIG. 3, which shows a section of wafer 210 that has been nanomachined to create a lamella 212, in preparation for sending the wafer to an ex situ plucker, such as device 110. Each lamella 212, including S/TEM viewing area 214, which is thinned sufficiently to be imaged by a S/TEM, is prepared in the nanomachining device 10, and left connected to the wafer by a pair of wafer-material tabs 216, defined in part by upwardly extending side cuts 218, so that the position of each lamellae remains fixed, prior to plucking.
Referring to FIGS. 4 and 5, the ex situ plucker 110 frees the lamella 212 though the use of a vacuum shaft 220 guided to the known position of each lamella, at a preset angle 224, adapted to provide optimum engagement with the known orientation of the lamella 212. The vacuum shaft 220, is moved into a position 220′, contacting the lamella 212 and is used to push and pull the lamella 212 until the tabs 216 (FIG. 3) break and then lifts it and places it into a lamella holding grid (not shown), for transport to a S/TEM device for imaging. Unfortunately, the expense of ex situ pluckers, plus the added complication of having to move the wafer and data from nanomachining device to ex situ plucker have limited the desirability of this solution.